This application is based on and claims priority under 35 U.S.C xc2xa7119 with respect to Japanese Patent Application 2000-384150, filed on Dec. 18, 2000 and Japanese Patent Application 2000-212992, filed on Jul. 13, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention:
The present invention relates to a rear-wheel steering angle control device which can improve the running stability of a vehicle with steering rear-wheels as well as front-wheels, in particular, a rear-wheel steering angle control device which can prevent the occurrence of the swing-out at the rear edge portion of the vehicle on condition that the rear-wheel is steered in counter direction to the front-wheel steering angle.
2. Description of the Related Art:
Hitherto, a rear-wheel steering angle control device is known which can improve the running stability of a vehicle and the capability of a small turning circle with steering rear-wheels as well as front-wheels. According to this rear-wheel steering angle control device, it becomes a subject of discussion of how the optimum target rear-wheel steering angle should be relative to the front-wheel steering angle corresponding to every driving condition of the vehicle. For example, in column xe2x80x9c7.5.3 4WS control for zero skidxe2x80x9d at page 196 of a publication (this publication will be hereafter referred to as xe2x80x9cpublication No.1xe2x80x9d) entitled xe2x80x9cSteering and Dynamicsxe2x80x9d (date of publication of the first edition: Sep. 10, 1996, publishing office: SANKAIDO, Inc., author: KAYABA KOUGYO, Inc.), the following is disclosed. The body slip angle (deviation angle between running direction of the vehicle and longitudinal direction of the vehicle) is focused on. When the vehicle speed is lower than a predetermined vehicle speed, the rear-wheel steering angle is controlled in a counter direction to the front-wheel steering angle in proportion to the front-wheel steering angle to obtain the capability of the small turning circle. Also when the vehicle speed is greater than a predetermined vehicle speed, the rear-wheel steering angle is controlled in the same direction as the front-wheel steering angle in proportion to the front-wheel steering angle in order to obtain the ideal vehicle dynamics which has the superior driving stability of the vehicle (this type of the control will be referred to as xe2x80x9cproportional steering angle controlxe2x80x9d hereafter).
However, when the proportional steering angle control as shown in publication No.1 is operated under a low range of the vehicle speed as the vehicle leaves, the rear-wheels are steered in a counter direction to the front-wheel steering angle. Therefore, the capability of the small turning circle is improved. However, there is a problem in that the rear edge portion of the vehicle (in particular, the corner portion of the rear edge of the vehicle) swings out and causes the vehicle to touch an obstacle at the side of the vehicle.
Therefore, in order to solve the problem of this swing-out at the rear edge portion of the vehicle and obtain the capability of the small turning circle as well, a means that controls the rear-wheel steering angle so that the rear edge portion of the vehicle follows the running locus of the front end point of the vehicle is proposed, for example, in column xe2x80x9c105 a reduction control means of swing-out of the 4WS vehiclexe2x80x9d of publication entitle (this publication will be referred to as xe2x80x9cpublication No.2 xe2x80x9d hereinafter) xe2x80x9cThe proceedings of Japanese Society of Automotive engineering 882, October, 1988xe2x80x9d. That is, the running locus of the front end point at every predetermined sampling interval is memorized as sample points, and the rear-wheel steering angle is controlled so that a running direction of the front end point at a nearest sample point which, is the nearest point to the then rear end point among the whole sample points, becomes equal to a running direction of the then rear end point.
However, in the case when this control means, as disclosed in publication No. 2, is employed, it is known that when a steering wheel is being steered just after the vehicle leaves the rear-wheel steering angle is controlled in the same direction to the front-wheel steering angle for a short while and subsequently is controlled in counter direction to the front-wheel steering angle (see the area C of FIG. 17). The occurrence of this matter causes a driver of the vehicle to experience a feeling of wrongness and also increases the energy needed to steer the rear-wheels with uselessly steering the rear-wheels extra.
Also, as for the usual 4WS vehicle, because the controllable range of the rear-wheel steering angle is smaller than the practicable range of the front-wheel steering angle, the rear edge portion of the vehicle can not always be forced to follow the running locus of the front end point of the vehicle. Therefore, when this control means as disclosed in the publication No.2 is applied, there is a problem such that the vehicle runs with the rear-wheels steered in the same direction as the front-wheel steering angle (a state where a deviation between the then rear end point and the nearest sample point remains continuously) under the low range of the vehicle speed, which deteriorates the turning performance of the vehicle (see FIG. 21). Hereinafter, this state will be referred to as xe2x80x9cthe crab-like running statexe2x80x9d.
In order to prevent the occurrence of this state as mentioned above, it is necessary to expand the controllable range of the rear-wheel steering angle to the same extent as the practicable range of the front-wheel steering angle. However, expanding the controllable range of the rear-wheel steering angle is difficult in view of the steering mechanism and the space, and causes deterioration of the steering feeling of the driver.
It is, therefore, an object of the present invention is to present a rear-wheel steering angle control device which can cope with both the improvement of the capability of the small turning circle and the reduction of the swing-out at the rear edge portion of the vehicle under a low range of vehicle speed which are contrary to each other, and does not provide a feeling of wrongness for the driver of the vehicle.
In order to achieve the object, there is provided a rear-wheel steering angle control device comprising a detecting means for detecting at least a vehicle speed, an actual front-wheel steering angle, and an actual rear-wheel steering angle; a target rear-wheel steering angle calculating means for calculating a target rear-wheel steering angle on the basis of the output of the detecting means and the target rear-wheel steering angle calculating means including a mileage detecting means for detecting a mileage reckoned from a leaving time of the vehicle; a rear-wheel steering angle control means for controlling the actual rear-wheel steering angle detected by the detecting means so that the actual rear-wheel steering angle becomes equal or approximately equal to the target rear-wheel steering angle, wherein the target rear-wheel steering angle calculating means calculates the target rear-wheel steering angle by means of a transitional control until the mileage reckoned from the leaving time reaches a predetermined mileage and calculates the target rear-wheel steering angle by means of a regular control after the mileage reckoned from the leaving time reaches the predetermined mileage, wherein the regular control is a control that the target rear-wheel steering angle is calculated in counter direction to the actual front-wheel steering angle at least when the vehicle speed is lower than a predetermined speed, and the transitional control is a control that a temporary target rear-wheel steering angle calculated on the assumption that the regular control is applied is normally employed as the target rear-wheel steering angle, and under a predetermined condition the target rear-wheel steering angle is calculated so that the target rear-wheel steering angle reaches to the temporary target rear-wheel steering angle gradually corresponding to increase of the mileage detected by the mileage detecting means.
According to this invention, the regular control is the control that the target rear-wheel steering angle is calculated in counter direction to the actual front-wheel steering angle at least when the vehicle speed is lower than the predetermined speed. Until the mileage reckoned from the leaving time reaches the predetermined mileage, the transitional control, which is different from the regular control is executed. This transitional control is the control that the temporary target rear-wheel steering angle calculated on the assumption that the regular control applied is the normally employed as the target rear-wheel steering angle, and under the predetermined condition the target rear-wheel steering angle is calculated so that the target rear-wheel steering angle reaches the temporary target rear-wheel steering angle gradually, corresponding to an increase of the mileage detected by the mileage detecting means.
Accordingly, with selecting xe2x80x9cthe predetermined mileagexe2x80x9d and xe2x80x9cthe predetermined conditionxe2x80x9d appropriately, even when the temporary target rear-wheel steering angle calculated, on the assumption that the regular control is applied, becomes large in a counter direction to the actual front-wheel steering angle, in the case that the steering wheel is being steered just after the vehicle leaves and the vehicle runs with less than the predetermined speed, the target rear-wheel steering angle reaches the temporary target rear-wheel steering angle gradually, corresponding to an increase of the mileage detected by the mileage detecting means. Therefore, the target rear-wheel steering angle can not reach the large temporary target rear-wheel steering angle without an increase of the mileage. Moreover, at this stage, the target rear-wheel steering angle is always calculated in a counter direction to the actual front-wheel steering angle.
The state cannot occur such that the rear-wheel steering angle is controlled in the same direction as the front-wheel steering angle for a short while even when the control means, as shown in the publication No. 2, is employed. Therefore, when the transitional control is executed once the mileage reckoned from the leaving time reaches the predetermined mileage according to this invention, the rear-wheel steering angle control device can be presented which can provide both the improvement of the capability of the small turning circle and the reduction of the swing-out at the rear edge portion of the vehicle even though they are contrary to each other, but does not provide a feeling of wrongness for the driver of the vehicle.
It is preferable that the target rear-wheel steering angle calculating means further includes a limited range setting means for setting a limited range of the target rear-wheel steering angle which expands gradually corresponding to an increase of the mileage detected by the mileage detecting means under the transitional control, wherein the transitional control is a control that the target rear-wheel steering angle is calculated within the limited range when the temporary target rear-wheel steering angle is out of the limited range. With setting the limited range of the target rear-wheel steering angle to expand gradually corresponding to an increase of the mileage, the target rear-wheel steering angle is controlled within the limited range without an increase of the mileage even when the temporary target rear-wheel steering angle mentioned above becomes large in a counter direction to the actual front-wheel steering angle. Therefore, with selecting n the limited range n appropriately, it becomes easier to well cope with both the improvement of the capability of the small turning circle and the reduction of the swing-out at the rear edge portion of the vehicle which are contrary to each other.
Moreover, in the case of setting the limited range of the target rear-wheel steering angle which expands gradually corresponding to increase of the mileage, it is preferable that the actual rear-wheel steering angle at the leaving time of the vehicle is employed as the target rear-wheel steering angle at the leaving time when the actual rear-wheel steering angle at the leaving time is out of the limited range, and until the target rear-wheel steering angle enters the limited range, the target rear-wheel steering angle calculated at one control cycle before is normally employed as the target rear-wheel steering angle. The temporary target rear-wheel steering angle is employed as the target rear-wheel steering angle only when the absolute value of the temporary target rear-wheel steering angle is smaller than the absolute value of the target rear-wheel steering angle. With this measure, when the actual rear-wheel steering angle at the leaving time is already out of the limited range, until the subsequent target rear-wheel steering angle enters the limited range, a constant value equal to the actual rear-wheel steering angle at the leaving time is normally employed directly as the subsequent target rear-wheel steering angle. The subsequent target rear-wheel steering angle is renewed to the temporary target rear-wheel steering angle whose absolute value is smaller than the absolute value of the target rear-wheel steering angle only when the absolute value of the temporary target rear-wheel steering angle is smaller than the absolute value of the target rear-wheel steering angle. Therefore, until the subsequent target rear-wheel steering angle enters the limited range, the absolute value of the target rear-wheel steering angle can not become larger than the absolute value of the actual rear-wheel steering angle at the leaving time. Herein, because the limited range of the target rear-wheel steering angle expands gradually corresponding to increase of the mileage, the target rear-wheel steering angle enters the limited range in due time with this continuous calculation. After the target rear-wheel steering angle enters the limited range, the temporary target rear-wheel steering angle is normally employed directly as the target rear-wheel steering angle and the target rear-wheel steering angle is calculated within the limited range only when the temporary target rear-wheel steering angle is out of the limited range. Therefore, with this measure, even when the actual rear-wheel steering angle at the leaving time is already out of the limited range, at the stage just after the vehicle leaves, the target rear-wheel steering angle cannot be calculated imprudently large in counter direction to the actual front-wheel steering angle. Accordingly, it becomes easier to wellcope with both the improvement of the capability of the small turning circle and the reduction of the swing-out at the rear edge portion of the vehicle which are contrary to each other.
It is also preferable that the transitional control is a control that the actual rear-wheel steering angle at the leaving time of the vehicle is employed as the target rear-wheel steering angle at the leaving time, and the subsequent target rear-wheel steering angle is calculated so that the subsequent target rear-wheel steering angle reaches gradually the temporary target rear-wheel steering angle calculated on the assumption that the regular control is applied in a predetermined ratio corresponding to increase of the mileage detected by the mileage detecting means. With this measure, even when the temporary target rear-wheel steering angle, calculated on the assumption that the regular control is applied, becomes large in a counter direction to the actual front-wheel steering angle when that the steering wheel is being steered just after the vehicle leaves and the vehicle runs with less than the predetermined speed, the target rear-wheel steering angle reaches to the temporary target rear-wheel steering angle gradually in the predetermined ratio corresponding to increase of the mileage detected by the mileage detecting means. Therefore, the target rear-wheel steering angle cannot reach the large temporary target rear-wheel steering angle without increased mileage. Moreover, at this stage, the target rear-wheel steering angle is always calculated in counter direction to the actual front-wheel steering angle. The state cannot occur in which the rear-wheel steering angle is controlled in the same direction as the front-wheel steering angle for a short while when the control means as shown in the publication No. 2 is employed. Therefore, as compared with the case when the regular control begins to be executed immediately the vehicle leaves, when the transitional control is executed until the mileage reckoned from the leaving time reaches the predetermined mileage according to this invention, the rear-wheel steering angle control device can be presented which can wellcope with both the improvement of the capability of the small turning circle and the reduction of the swing-out at the rear edge portion of the vehicle (which are contrary to each other) and which does not provide a feeling of wrongness for the driver of the vehicle.
By the way, these descriptions mentioned above can be presented on the assumption that the mileage detected by the mileage detecting means is obtained continuously as the vehicle runs. However, in the case that the mileage detected by the mileage detecting means is obtained discretely at every predetermined pulse input time as the vehicle runs, the limited range of the target rear-wheel steering angle and the target rear-wheel steering angle as mentioned above, which are calculated according to the mileage detected by the mileage detecting means, are also calculated discretely. In this case, it is preferable that the discrete values of the limited range of the target rear-wheel steering angle and the target rear-wheel steering angle are revised to be continuous values by bridging them as follows.
Namely, as for the limited range of the target rear-wheel steering angle, it is preferable that the limited range setting means further includes a continuous limited range calculating means for calculating a continuous limited range which is the continuous value bridged at least on the basis of discrete values of the limited range calculated at past pulse input times, wherein the continuous limited range calculating means calculates a current continuous limited range from a latest pulse input time to a next pulse input time at least on the basis of the discrete values of the limited range calculated at past pulse input times and the continuous limited range at a predetermined time before the latest pulse input time, and the continuous limited range calculating means employs the current continuous limited range as the limited range.
Also, as for the target rear-wheel steering angle, it is preferable that the target rear-wheel steering angle calculating means further includes a continuous target rear-wheel steering angle calculating means for calculating a continuous target rear-wheel steering angle which is the continuous value bridged at least on the basis of discrete values of the target rear-wheel steering angle calculated at past pulse input times, wherein the continuous target rear-wheel steering angle calculating means calculates a current continuous target rear-wheel steering angle from a latest pulse input time to a next pulse input time at least on the basis of the discrete values of the target rear-wheel steering angle calculated at past pulse input times and the continuous target rear-wheel steering angle at a predetermined time before the latest pulse input time, and the continuous target rear-wheel steering angle calculating means employs the current continuous target rear-wheel steering angle as the target rear-wheel steering angle.
Accordingly, when the discrete values of the limited range of the target rear-wheel steering angle and the target rear-wheel steering angle are revised to be continuous values, the current continuous value from the latest pulse input time to the next pulse input time is calculated on condition that the revised continuous value at the predetermined time before the latest pulse input time as well as the discrete values calculated at past pulse input times, is taken into consideration. Therefore, the current continuous value obtained by the revised calculation is expected to become a reliable value which takes a past tendency of increase and decrease and a past deviation between the discrete value and the revised continuous value into consideration.
Also, in order to achieve the object of the present invention mentioned above, there is provided a rear-wheel steering angle control device comprising a detecting means for detecting at least a vehicle speed, an actual front-wheel steering angle, and an actual rear-wheel steering angle; a target rear-wheel steering angle calculating means for calculating a target rear-wheel steering angle on the basis of the output of the detecting means; a rear-wheel steering angle control means for controlling the actual rear-wheel steering angle detected by the detecting means so that the actual rear-wheel steering angle becomes equal or approximately equal to the target rear-wheel steering angle, wherein the target rear-wheel steering angle calculating means includes a front end point locus memory means in which a front side point of two points which are both ends of a line segment parallel to a longitudinal direction of the vehicle is employed as a front end point and a rear side point of the two points is employed as a rear end point, and the running locus of the front end point at every predetermined sampling interval is memorized as sample points, and the target rear-wheel steering angle calculating means includes a target rear-wheel steering angle limiting means in which the target rear-wheel steering angle is limited so that the rear end point does not deviate from an inside area of the running locus of the front end point memorized by the front end point locus memory means. Herein, xe2x80x9can inside area of the running locus of the front end point memorized by the front end point locus memory meansxe2x80x9d refers to a right side area of the running direction of the running locus of the front end point when the vehicle turns to a right side of the running direction and refers to a left side area of the running direction of the running locus of the front end point when the vehicle turns to a left side of the running direction.xe2x80x9c[A] front end pointxe2x80x9d and xe2x80x9ca rear end pointxe2x80x9d can be located at any position of the vehicle on the condition that these two points are both ends of a line segment parallel to the longitudinal direction of the vehicle.
According to this invention, the target rear-wheel steering angle calculating means includes the target rear-wheel steering angle limiting means in which the target rear-wheel steering angle is limited so that the rear end point does not deviate from the inside area of the running locus of the front end point memorized by the front end point locus memory means. Therefore, when the rear-wheels are steered in a counter direction to the front-wheel steering angle, the rear end point cannot swing out toward the outside area of the running locus of the front end point which solves the problem of the swing-out of the rear end point of the vehicle.
Also, according to this invention, the target rear-wheel steering angle calculating means can calculate the target rear-wheel steering angle in any way on the condition that the target rear-wheel steering angle calculating means calculates the target rear-wheel steering angle based on the output of the detecting means, which detects at least the vehicle speed, the actual front-wheel steering angle, and the actual rear-wheel steering angle. And the target rear-wheel steering angle limiting means limits the target rear-wheel steering angle so that the rear end point does not deviate from the inside area of the running locus of the front end point memorized by the front end point locus memory means in the case that the rear end point deviates from the inside area with the vehicle running on the basis of the calculated target rear-wheel steering angle. Therefore, this target rear-wheel steering angle calculating means is essentially different from the control means as shown in the publication No. 2 in which the rear-wheel steering angle is controlled so that the rear end point of the vehicle is always forced to follow the running locus of the front end point of the vehicle.
Accordingly, when this invention is applied to the 4WS vehicle in which the controllable range of the rear-wheel steering angle is smaller than the practicable range of the front-wheel steering angle, the crab-like running state cannot occur. Further, the rear-wheel steering angle control device which can well cope with both the improvement of the capability of the small turning circle and the reduction of the swing-out at the rear edge portion of the vehicle (which are contrary to each other) without the deteriorations of the turning performance and other performances of the vehicle.
It is preferable that the target rear-wheel steering angle limiting means calculates a deviation between the then rear end point and a nearest sample point which is the nearest point to the then rear end point among the whole sample points, and calculates a limited target rear-wheel steering angle so that the absolute value of the limited target rear-wheel steering angle is the value made by adding a tolerable amount, calculated corresponding to the deviation to the absolute value of a rear-wheel steering angle calculated so that a running direction of the front end point at the nearest sample point becomes equal to a running direction of the then rear end point. With this measure, it is possible to prevent the swing-out of the rear end point of the vehicle securely and calculate the limited target rear-wheel steering angle more appropriately. In particular, because the absolute value of the limited target rear-wheel steering angle is the value made by adding the tolerable amount, which is calculated corresponding to the deviation, when the deviation occurs, the absolute value of the limited target rear-wheel steering angle is calculated to be larger by the tolerable amount. Therefore, when the rear-wheels are steered in a counter direction to the front-wheel steering angle, it is possible to further improve the capability of the small turning circle.
Moreover, it is preferable that the front end point locus memory means employs a front edge portion of the vehicle as the front end point and employs a rear edge portion of the vehicle as the rear end point. With this measure, the rear edge portion of the vehicle cannot swing out toward the outside area of the running locus of the front edge portion of the vehicle. Therefore, it is possible to prevent the state that the rear edge portion of the vehicle swings out and touches an obstacle at the side of the vehicle.